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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_config.build_config"></a><a class="link" href="build_config.html" title="Build Time Configuration">Build Time Configuration</a>
</h2></div></div></div>
<p>
      There are times when you want to control whether a build target gets built
      or not, based on what features the compiler supports. For example, suppose
      you have a test file "test_constexpr_128.cpp" which requires three
      key features in order to build:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          The <code class="computeroutput"><span class="keyword">constexpr</span></code> keyword as detected
          by BOOST_NO_CXX11_CONSTEXPR.
        </li>
<li class="listitem">
          User defined literals, as detected by BOOST_NO_CXX11_USER_DEFINED_LITERALS.
        </li>
<li class="listitem">
          The <code class="computeroutput"><span class="identifier">__int128</span></code> data type,
          as detected by BOOST_HAS_INT128.
        </li>
</ul></div>
<p>
      Clearly we know that if these features are not supported by the compiler, then
      there's simply no point in even trying to build the test program. The main
      advantages being:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          Faster compile times - build configuration uses lightweight tests the results
          of which are also cached.
        </li>
<li class="listitem">
          Less noise in build output - there's no reason to be faced with pages of
          template instantiation backtrace if we know the file can never compile
          anyway.
        </li>
<li class="listitem">
          Less noise in the online test results - the test will show up as blank,
          rather than as a fail in the online test matrix.
        </li>
<li class="listitem">
          A better experience for end users building all of Boost, if those libraries
          which can not be built for the current target compiler are simply skipped,
          rather than generating pages of error output.
        </li>
</ul></div>
<p>
      Returning to our example, the test case is probably executed in it's Jamfile
      via the "run" rule:
    </p>
<pre class="programlisting"><span class="identifier">run</span> <span class="identifier">test_constexpr_128</span><span class="special">.</span><span class="identifier">cpp</span> <span class="special">;</span>
</pre>
<p>
      We now need to make this target conditional on the necessary features. We can
      do that by first importing the necessary rule at the start of the Jamfile:
    </p>
<pre class="programlisting"><span class="identifier">import</span> <span class="identifier">path</span><span class="special">-</span><span class="identifier">to</span><span class="special">-</span><span class="identifier">config</span><span class="special">-</span><span class="identifier">lib</span><span class="special">/</span><span class="identifier">checks</span><span class="special">/</span><span class="identifier">config</span> <span class="special">:</span> <span class="identifier">requires</span> <span class="special">;</span>
</pre>
<p>
      Assuming that the test case is in the usual directory:
    </p>
<pre class="programlisting"><span class="identifier">libs</span><span class="special">/</span><span class="identifier">yourlib</span><span class="special">/</span><span class="identifier">test</span>
</pre>
<p>
      then the import rule will actually be:
    </p>
<pre class="programlisting"><span class="identifier">import</span> <span class="special">../../</span><span class="identifier">config</span><span class="special">/</span><span class="identifier">checks</span><span class="special">/</span><span class="identifier">config</span> <span class="special">:</span> <span class="identifier">requires</span> <span class="special">;</span>
</pre>
<p>
      Then add a "requires" rule invocation to the requirements section
      of the target:
    </p>
<pre class="programlisting"><span class="identifier">run</span> <span class="identifier">test_constexpr_128</span><span class="special">.</span><span class="identifier">cpp</span>
   <span class="special">:</span> <span class="special">:</span> <span class="special">:</span> <span class="special">#</span><span class="identifier">requirements</span><span class="special">:</span>
   <span class="special">[</span> <span class="identifier">requires</span> <span class="identifier">cxx11_constexpr</span> <span class="identifier">cxx11_user_defined_literals</span> <span class="identifier">int128</span> <span class="special">]</span> <span class="special">;</span>
</pre>
<p>
      Notice that multiple arguments can be added to the requires rule, and that
      these are always the same as the Boost.Config macro name, but in lower case
      and with the <span class="emphasis"><em>boost_no_</em></span> or <span class="emphasis"><em>boost_has_</em></span>
      prefix removed. You can also use any C++ standard feature-macro name with the
      leading underscores removed (see more below).
    </p>
<p>
      When building the above example, you will see at the start of the build process
      the results of the configuration, for example GCC in C++11 mode gives:
    </p>
<pre class="programlisting"><span class="special">-</span> <span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Config</span> <span class="identifier">Feature</span> <span class="identifier">Check</span><span class="special">:</span> <span class="identifier">int128</span> <span class="special">:</span> <span class="identifier">yes</span>
<span class="special">-</span> <span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Config</span> <span class="identifier">Feature</span> <span class="identifier">Check</span><span class="special">:</span> <span class="identifier">cxx11_constexpr</span> <span class="special">:</span> <span class="identifier">yes</span>
<span class="special">-</span> <span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Config</span> <span class="identifier">Feature</span> <span class="identifier">Check</span><span class="special">:</span> <span class="identifier">cxx11_user_defined_literals</span> <span class="special">:</span> <span class="identifier">yes</span>
</pre>
<p>
      If you wish to make a build conditional on a C++ standard feature macro then
      you can specify these too, just remove the leading underscores from the name.
      For example:
    </p>
<pre class="programlisting"><span class="special">[</span> <span class="identifier">requires</span> <span class="identifier">cpp_constexpr</span> <span class="special">]</span>
</pre>
<p>
      To require C++11 style const-expressions. If you want to specify a macro from
      a particular standard, then you append an underscore followed by the (2 digit)
      year of the standard, for example:
    </p>
<pre class="programlisting"><span class="special">[</span> <span class="identifier">requires</span> <span class="identifier">cpp_constexpr_17</span> <span class="special">]</span>
</pre>
<p>
      For C++17 constepxr. If you don't specify a standard then you get the first
      version that introduced the macro. In addition there are only standard-specific
      rules for each version bump of the macro, so:
    </p>
<pre class="programlisting"><span class="special">[</span> <span class="identifier">requires</span> <span class="identifier">cpp_if_constexpr_17</span> <span class="special">]</span>
</pre>
<p>
      Is fine since the macro was introduced in C++17 and is the same as the un-versioned
      name, but:
    </p>
<pre class="programlisting"><span class="special">[</span> <span class="identifier">requires</span> <span class="identifier">cpp_if_constexpr_20</span> <span class="special">]</span>
</pre>
<p>
      Will result in a build error since there is no C++20 version bump for <code class="computeroutput"><span class="identifier">__cpp_if_constexpr</span></code>.
    </p>
<p>
      That's all there is to this handy feature, should at any time you be unsure
      of the feature-test names you can pass to the "requires" rule, then
      search for the Boost.Config macro of interest in libs/config/checks/Jamfiles.v2,
      and the name of the feature check will follow it.
    </p>
<p>
      And finally, this feature is built around the Boost.Build built in rule <span class="emphasis"><em>check-target-builds</em></span>
      which can be used to perform more generalized build-time feature testing. The
      checks in this library are provided as a convenient shorthand without the need
      for you to write the test cases yourself.
    </p>
</div>
<div class="copyright-footer">Copyright © 2001-2007 Beman Dawes, Vesa Karvonen, John
      Maddock<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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